The present invention relates to the preparation of halogenated phenolic compounds. More particularly the present invention relates to an improved process for the direct halogenation of phenolic compounds whereby reduced amounts of by-products, particularly halogenated dioxin compounds are also prepared.
Halogenated phenols such as chlorinated phenols, especially tetrachlorophenols and pentachlorophenol, are valuable fungicides that are conveniently prepared by the direct halogenation of phenol or halogenated phenols in the presence of various catalysts such as AlCl.sub.3, FeCl.sub.3, activated carbon and quinoline, tellurium and various salts thereof, as well as such catalysts in combination with various promoters such as those taught in U.S. Pat. No. 4,376,220. The preferred catalyst is aluminum trichloride.
In the case of chlorinated phenols, the reaction is conducted neat at a starting temperature of about 65.degree. C.-130.degree. C. After three or four chlorine atoms have been substituted onto the phenol, the temperature of the reaction mixture is increased to maintain a temperature approximately 10.degree. C. above the melting point of the chlorinated mixture. The reaction is normally complete in 5-15 hours. Because the mixture's melting point increases during the later stages of the chlorination, the preparation of pentachlorophenol may require reaction temperatures of up to about 190.degree. C. or even higher so that the reaction mixture remains in a liquid state. It is now known that elevated temperatures necessary for the preparation of highly chlorinated phenols are particularly conducive to the formation of increased amounts of undesirable by-products particularly chlorinated dibenzo-p-dioxins.
In U.S. Pat. No. 2,131,259, it is further taught to employ solvents such as chlorinated aliphatic compounds in the reaction, thereby allowing the use of lower reaction temperatures while maintaining a liquid reaction mixture. Suitable solvents included ethylene dichloride, sym-tetrachloroethane, trichloroethane, carbon tetrachloride and propylene chloride. The use of such solvents has not proven to be practical in commercial usage due to the possible formation of solvent-reactant by-products as well as the increased production costs associated with removal and recycle of solvent during the process, and decreased effectiveness of the chlorination process due to the solvent's dilution effect.
It would be desirable to provide a process that is capable of preparing highly halogenated phenolic derivatives without concomitant formation of substantial amounts of by-products. It would further be desirable to provide a process that would-allow the preparation of highly halogenated phenolic compounds at reduced temperatures without the use of a solvent.